Italy suffered a stunning 3-2 quarter-final defeat against 10-man Belgium in the Olympic soccer on Saturday while Brazil and titleholders Argentina both won in extra-time to set up a semi-final meeting.
Lionel Messi set up one goal and scored the other in Argentina's 2-1 win over the Netherlands and Brazil beat Cameroon 2-0 to avenge their traumatic quarter-final defeat against the Africans in Sydney eight years ago.
Belgium, who will face Nigeria in the semi-finals, found themselves trailing 1-0 and down to 10 men after 18 minutes when Thomas Vermaelen was sent off for dragging down Giuseppe Rossi, who scored from the spot.
Moussa Dembele equalised with a 24th minute header which was deemed to have crossed the line despite Luca Cigarina's block and Kevin Miralles fired Belgian ahead in first half injury-time.
Rossi equalised with another penalty in the 74th minute before Dembele struck the winner in the 80th minute as Italy uncharacteristically lost their defensive cool.
"Everybody is very important in my team but you need players who can make the difference and today Moussa Dembele made the difference," Belgium coach Jean Francois De Sart said.
Argentina went ahead with Messi's second goal of the Games but Otman Bakkal equalised before halftime.
Messi set up the winner in the first half of extra-time when he sliced open the Dutch defence and found Angel Di Maria, who side-footed home to make up for his double penalty-miss against Serbia.
Brazil and Cameroon produced one of the tournament's low points as they shared 12 yellow cards and 56 fouls in a cynical game which also saw Cameroon's Albert Baning dismissed in the 52nd minute for a second bookable offence.
Rafael Sobis and Marcelo scored in a four-minute spell in extra-time to keep Brazil -- jeered by the crowd -- on course for a first Olympic gold.
"It's not even fun to talk about the referees any more," said Brazil coach Dunga. "When you have players of this level and officials who make the sort of decisions we saw today ... that says it all."
The match was watched by FIFA president Sepp Blatter who spoke too soon when he praised the controversial tournament, which is restricted to under-23 teams with three over-age players allowed per side.
"The standard of football at the Olympics has never been higher," he told reporters before kick off.
Nigeria, publicly criticised by coach Samson Siasia for ball-hogging in their last match, beat Ivory Coast 2-0.
In a match played to the backing of the off-key trumpets of the Nigerian supporters' club, Peter Odemwingie swept home Victor Obinna's pass in the 44th minute and Obinna converted an 82nd minute penalty.